The present invention relates to fluid energy mills, in particular, to an improved fluid energy mill which is provided with a fluid dynamic control insert that maintains or improves the quality of the milled product at lower energy consumption and at lower cost of operation.
Fluid energy mills of a vortex type are well known and widely employed in certain industries because of their efficiency and economy in comminution of particulate solids. A number of early designs are described in considerable detail in Andrews, U.S. Pat. No. 2,032,827. These mills generally comprise a disc-shaped zone wherein an inward circular or spiral flow of the gaseous fluid causes attrition of the particles at the periphery and provides a size separation in an intermediate zone. The mill combines the function of grinding and classification within a single chamber. Since the fluid is fed into the periphery and discharged at the axis of a vortex, there is a tendency for particles to be swept toward the central outlet in a spiral path. The force due to drag of the fluid acting on the suspended particle is opposed by the centrifugal force. This balance of forces can be so adjusted that coarse particles tend to return to or be held at the periphery for more attrition while smaller particles are swept to the center for collection in a cyclone and/or filters. In these mills, the energy for comminution is supplied in a gaseous fluid medium injected tangentially into the vortex chamber to create and maintain the vortex.
Attempts to prevent premature escape of larger particles from the mill or to avoid energy loss have been described in the literature. For example, Doyle and Becker, U.S. Pat. No. 3,425,638 describe a fluid energy mill having a cylindrical baffle being closed at one end and having a plurality of passageways on the cylinder surface. Taylor, U.S. Pat. No. 4,219,164 describes a fluid energy mill with an upwardly flowing vortex having a circular annulus. Although various modifications of the above-described mills have been proposed, none has proven to be wholly satisfactory and further improvements are desirable.
Trost, U.S. Pat. No. 2,562,753 discloses a fluid energy mill which has a plurality of restrictors positioned within the mill such that the restrictors are adjacent to and in line with the grinding fluid jets. The restrictors are located such that a confined passageway is created, wherein and whereby material is forced closer to the jets to increase the cutting action of the jets upon the particles of the material. Thus, these restrictors have a negative angle of attack which is further discussed below. The fluid from the jets deflects some of the material against the restrictors which results in some abrasion of the material. It is suggested that this mill can be used to grind materials such as powdered milk, cocoa, stock feed and instant coffee.
In the white pigment industry, there is a particular need to reduce the amount of oversized material passing prematurely into the resulting pigment product. Thus, the intensity of grinding is typically increased with resultant greater costs in terms of fluid use, energy consumption, and reduced capacity per nill. Further, with such processes, the amount of oversized material may be reduced, but there may be adverse effects on pigment properties. Further enhancement in grinding efficiency is needed. Concomitantly, there is a need to achieve a longer life of the inner wear liners typically used within these mills. The present invention meets these needs.